This invention relates to an optical glass having refractive indices (nd) of about 1.770 to about 1.875 and Abbe value (.nu.d) of about 37.5 to about 28.5 and consisting basically of a B.sub.2 O.sub.3 - La.sub.2 O.sub.3 - ZrO.sub.2 - Nb.sub.2 O.sub.5 - PbO - BaO (and/or ZnO) system. As compared with known optical glasses having the above mentioned high refractive indices, the optical glass according to the invention is so excellent in its light transmission property in a transition region from an ultraviolet portion to a visible portion of light that the optical glass is substantially free from undesirable colouring.
Known in the art of the optical glass having above described optical properties are ones consisting of a SiO.sub.2 - B.sub.2 O.sub.3 - La.sub.2 O.sub.3 - ZrO.sub.2 - ThO.sub.2 - TiO.sub.2 - BaO - Nb.sub.2 O.sub.5 system as disclosed in British Patent No. 1235206, a B.sub.2 O.sub.3 - SiO.sub.2 - La.sub.2 O.sub.3 - TiO.sub.2 - CdO system as disclosed in German Patent No. 1147359, a B.sub.2 O.sub.3 - La.sub.2 O.sub.3 - ThO.sub.2 - ZrO.sub.2 - PbO - ZnO system as disclosed in German Patent No. 958150, a SiO.sub.2 - La.sub.2 O.sub.3 - ZrO.sub.2 (and/or Ta.sub.2 O.sub.5) - BaO system as disclosed in Japanese Laid-Open Publication No. 73914/1975 and a B.sub.2 O.sub.3 - SiO.sub.2 - La.sub.2 O.sub.3 - ZrO.sub.2 (and/or Ta.sub.2 O.sub.5) - ZnO as disclosed in Japanese Laid-Open Publication No. 53413/1975. The optical glass including CdO or ThO.sub.2 is poisonous, whereas the optical glass containing a large amount of SiO.sub.2 or ZnO without including CdO or ThO.sub.2 has some difficulty in melting. The optical glass not containing a large amount of SiO.sub.2 or ZnO but containing a relatively large amount of TiO.sub.2 is extremely low in light transmission in the transition region from an ultraviolet portion to a visible portion with a resulting strong tendency to colouring which causes a serious disadvantage in an optical design.